In the formation of full-color fixed images, since four colors of toners are usually used, the influence of an external additive freed from the toner on an organic photoconductor (OPC) or the like is big, and background fog is likely to be generated under high-temperature and high-humidity, so that degradation of the fixed images is likely to be caused, as compared to the development using only one color of a toner. Therefore, it has been proposed that the external additive is solidly adhered to a toner surface. For example, JP2004-126005 A proposes a method of carrying out the surface treatment with the external additive on a toner of which a BET specific surface area or the like is specified, and JP2003-215838 A proposes a method of fixing the external additive under compressed shearing stress conditions.